The FMS concept usually covers the functions of positioning, preparing flight plans, and guidance.
Positioning serves continuously to determine where the aircraft is in three-dimensional space (latitude, longitude, and altitude). Implementing this function makes use of a set of sensors continuously delivering a state vector of the aircraft, including sensors for ground speed, rate of turn, trim and heading, and air speed.
In a conventional FMS, a flight plan is essentially a succession of rectilinear segments that are horizontal, i.e. without any vertical dimension.
The preparation of flight plans (PDVs) and their storage generally covers: i) databases containing PDVs and the points constituting the PDVs; ii) optionally, means for loading and transferring said databases to or from preparation means on the ground (removable storage modules); iii) means for editing said databases (a man-machine interface based on a control and display unit (CDU), for example); and iv) means for selecting one PDV—from amongst all those in the database—for input to the guidance function. This PDV is referred to as the “destination flight plan”. This PDV is also usually displayed on a chart background.
Guidance consists in evaluating the differences between the aircraft state vector and the PDV that has been selected as the destination PDV. This function generates instructions for the automatic flight control system (AFCS) or autopilot (AP) and for the human pilot, enabling the aircraft to be kept on the flight plan.
The navigation systems that are presently in operation can be classified in two categories: visual flight rules (VFR) and civil instrument flight rules (civil IFR).
VFR navigation systems can define flight plans in two dimensions (2D) only. The flight plan is constituted by an ordered list of geographical points defined solely by their latitudes and longitudes (no altitude information). To construct a flight plan, the pilot can select points in the database, or can create these points if so desired.
In VFR systems, the guidance function serves to ensure that the horizontal trace of the aircraft trajectory is as close as possible to rectilinear segments interconnecting successive points in pairs. There is no management of altitude by the navigation system. It is the pilot who selects and maintains flying altitude (making use, where appropriate, of the “acquire and hold altitude” mode of the AP). The altitude selected for the flight can be anticipated by visually examining a digital chart (including terrain height information) if there is one on board the aircraft, or else a paper chart. Ultimately, maintaining separation from the terrain and obstacles is a task that is achieved visually by the pilot.
Civil IFR navigation systems enable flight plans to be managed in three dimensions (3D). Under such circumstances, the construction of a flight plan is founded on a database of “branches”, i.e. horizontal rectilinear segments and distance measurement equipment arcs (DME arcs) which are used only rarely, and solely in the terminal stage prior to the final approach. Each segment is associated with a minimum flying altitude and the necessary data concerning lateral and vertical localization and guidance.
The database is closed and certified. The term “closed” means that the pilot cannot change its content. The term “certified” means that the database DB has been constructed by the appropriate aviation authority which guarantees adequate separation between the branches and the terrain and obstacles. Furthermore, all necessary precautions are taken to guarantee the integrity of its content on board the aircraft.
Constructing an IFR flight plan consists in selecting an itinerary from a network of “air routes”.
Such an IFR navigation system satisfies the needs of commercial transport planes, but it impedes performing certain missions that aircraft are required to perform, in particular those of civilian helicopters which present special characteristics.
The transit stage of a helicopter mission is usually performed at lower altitudes than those used by commercial transport planes, in order to avoid interfering with them. Flying at low altitude also presents numerous other advantages: shorter mission times; fuel savings; better comfort for passengers and crew since helicopter cabins are not pressurized. This point is particularly crucial when transporting certain injured persons who cannot tolerate any variation in pressure.
Helicopters usually take off and land away from airport structures. Mention can be made, for example, of emergency medical service (EMS) missions where the landing point might be the roof of a hospital or a completely unprepared area such as a football pitch or a field situated near an accident.
Most helicopter missions are emergency missions (this applies particularly to EMS missions), which puts severe constraints on the opportunities for meticulous preparation being performed on the ground.
Even when the emergency is not as great as that for an EMS mission, helicopter missions are often varied and specific, unlike the regular routes followed by commercial air transport; therefore, as a general rule, flight plans cannot be reused several times over, which means that a ground team dedicated to preparing missions is unlikely to be economically viable.
Because of these specific features, most helicopter missions can be undertaken only under conditions of good visibility, known as visual meteorological conditions (VMC). It is estimated that close to 40% of civilian helicopter missions are prevented because of poor weather conditions.
The invention seeks to provide a helicopter with “stand-alone” capacity for IFR flight, i.e. not relying on air route networks dedicated to commercial airliners and away from airport infrastructures. Another object is to increase the reactivity that is frequently required of a helicopter crew.
U.S. Pat. No. 6,421,603 (Pratt et al.) describes a method of evaluating the risks of interference between an intended flight plan and obstacles, in which the flight plan is defined in the form of a coarse trajectory made up of a sequence of segments having parameters defining their extent in three dimensions (horizontally and vertically); a route generator converts those segments and parameters into parallelepipeds or polygons in order to constitute a route model; stationary obstacles are represented in the form of terrain rectangles having altitudes, and in the form of terrain rectangles subdivisions, while moving obstacles are modeled by means of segments, in a manner similar to the flight plan. Interference is detected by comparing the respective models for the itinerary and the obstacles; an alarm is triggered when interference is detected.
That system does not enable the pilot to be shown the portion of the trajectory that corresponds to the detected interference; nor does it make it possible to manage in real time a precise predicted trajectory that is likely to be followed very closely by a rotary wing aircraft operating at low altitude.
U.S. Pat. No. 6,424,889 (Bonhoure et al.) describes a method of generating a horizontal trajectory for avoiding zones that are dangerous for an aircraft; the method comprises determining circles that are tangential to the trajectory at an initial point and at a final point; determining tangents to the circles and to models of the dangerous zones; selecting pairs of tangents that define a skeleton trajectory, and determining a 2D trajectory comprising circles interconnecting the tangents; that method does not enable on-board personnel to intervene in planning the itinerary.